If you have installed, say, XP, on a Vista machine, in the Boot Menu, you may no longer be able to see Vista as an option. This occurs because when a different version of Windows is installed, it rewrites the MBR to call its own boot loader.
So if you install Vista on a PC where XP is already installed, the Vista boot menu incorporates the options from the older OS’s boot menu. But, on the other hand, if you install XP on Vista, it will overwrite the MBR with one that does not recognize the newer Vista’s Boot Loader.
To resolve this issue, open cmd in XP.
Where is the Drive letter.
Reboot. You will now be able to see the Vista menu option/s.
Now To restore the entry for XP, open an elevated command prompt and enter this: